2-way
valve controlled by a solenoid. Depending on the wishes of the ECU
it can be wide open, shut or pulsed open. thus fine-tuning the airflow
at the back of its actuator.

WGV

Waste
Gate Valve

Located
upstream of turbo#1 it can partially control boost during low-rpm
operation and fully control boost during high-rpm operation.

It
cracks open at 11-12psi and is fully open at 18psi. According to
Toyota the diaphram gets damaged at over 27psi.

The
ECU pulses it open depending on boost pressure, throttle position
and engine speed (rpm).

EGCV

Exhaust
Gas Control Valve

Located
downstream of turbo#2 it controls the flow of exhaust gases via
the 'hot' side of turbo #2

It
is activated some time between 3 and 4Krpm depending on engine speed
(rpm), vehicle speed, intake air volume and intake air temperature.

IACV

Inlet
Air Control Valve

Located
downstream of turbo#2 it controls the flow of boost pressure via
the 'cold' side of turbo #2

It
needs 7psi to fully extend. The ECU activates it based on engine
speed (rpm), vehicle speed and throttle position.

EBV

Exhaust
Bypass Valve

Located
upstream of turbo#2 it controls the flow of exhaust gases much like
the EGCV, which is much bigger though.

During
low-rpm operation it partially controls boost along with the WGV.During
mid-rpm operation it also controls the pre-spool of turbo#2During
high-rpm operation it cannot control anything, since the wide-open
EGCV steals all the 'hot gas' airflow of turbo#2 anyway.

Think
of the EBV as a small-capacity alternative to the EGCV, only useful
for the transition period. The ECU pulses it open depending on boost
pressure, throttle position and intake air volume. (Note the absense
of engine speed in the mix)

According
to Toyota it fully extends at 14 psi. (softer spring compared to
the WGV). But measuring it shows that it also cracks open at 11psi
and is fully open at 17psi, go figure...

Reed
Valve

Located
just before the IACV it allows the IACV to be bypassed when it is
still closed despite turbo#2 making some boost (mid-rpm operation)

Technically
it is a simple, passive device that only flows one-way. Hence it
needs no actuator of VSV.

Think
of the reed valve as a small-capacity alternative to the IACV, only
useful for the transition period.

Pressure
Tank

It
holds pressurised air from the intake so that the IACV and EGCV
actuators can be pressurised

There are
two effective wastegate valves at work: the WGV upstream of turbo #1
and the EBV, also upstream of turbo #1.

2nd
Movement: Molto vivace --- Mid-rev operation (3500-3800rpm)

(Also
known as 'prespooling' stage)

EGCV and
IACV are still shut, but turbo #2 is not completely asleep.

The
ECU is pulsing the WSV and EBV VSVs, gradually pressurising the
actuators.

As
a result the EBV is now cracking open, so there is a trickle of
exh gases via the exhaust bypass pipe through the turbine of #2
which is now freewheeling.

Looking
closely we can see that this flow is because of the pressure differential
between the cooled-down and fast-moving (ie lower-pressure) gases leaving
the #1 turbine and the full output of turbine #2, most of it still has
no choice but to make its way via stage exit left (the exhaust 'connector'
concertina pipe and exit via turbine #1)

3rd
Movement: Adagio --- Mid-rev operation (3800-4000rpm)

(second
turbo coming on boost)

EGCV
is opened by the ECU:

Its
actuator is forced open as the EGCV VSV opens and boost pressure
rushes from the pressure tank

The
IACV is still shut, so any boost turbo#2 produces joins the rest
of the crew via the reed valve.

It can only spin so much with it's intake being in deep vacuum
(still throttled by the IACV)

Fourth
Movement: Fugal finale --- High-rev operation (4000+ rpm)

(second
turbo full on boost)

The EGCV
has already been opened by the ECU by now

Very
soon afterwards the IACV is also opened by the ECU:

Its
actuator also forced open as the IACV VSV opens and boost pressure
rushes from the pressure tank

Turbo #2
now produces full boost and it's airflow joins the output of #1 via
the wide "Y" intake join.

At this
stage the whole system is symmetrical:
Cylinders 1,2,3 push their exhaust gases straight into turbine#1 (the
exhaust ports are aligned accordingly).
Cylinders 4,5,6 push their exhaust gases straight into turbine#2 (their
exhaust ports are also aligned accordingly as shown below).

The flow
via the 'connector' exhaust pipe is now virtually zero, since there
is no pressure difference across it.

The reed
valve is now also deprived of airflow, since the wide open IACV eliminates
any pressure difference across it.

The EGBV
is also irrelevant now, since it also is deprived of airflow (courtesy
of the wide open EGCV)

The only
way for the ECU to enforce boost control for the whole system now is
via the WGV of turbo#1. This actuator is always pressurised with
boost gases --- in contrast to the IACV and EGCV which are only pressurised
from the pressure tank whenever their VSVs allow it.

In fact
the WGV has a lot in common with the EBV:

1. their
actuators are both being fed continously boost pressure from the intake
(and not the pressure tank)
2. their VSVs are further downstream from their boost source. Under
low load operation that boost is being bled away at the intake right
after the MAF meter (since this is metered air, i.e. it has been accounted
for so the ECU has already put aside fuel for it). Both of these 'relief'
paths meet at the top of the engine, where low-rev boost control can
be affected, but more on this later.
3. Under high load operation boost is kept inside these actuators, as
the VSVs are pulsed closed by the ECU.

As high
and low-rev boost control is down to these two valves, it makes sense
that they share so many operational characteristics.

The other
'pair' of similar valves is the EGCV and IACV combo. These share the
following:1. they
are both shut at their idle state (forcing turbo #2 to stay asleep)
2. they both have their actuators pressurised by the pressure tank,
and not directly by the boost 'signal'.
3. they are not pulsed open by the ECU, rather forced wide-open by the
full force of the pressure tank.
4. they both have their VSVs upstream of their boost source (the pressure
tank)

It
is important to understand the whole sequential operation, or else any
attempt to modify boost control can end in tears. The frequent turbocharger
failures of 'boosted' supras are testament to such poorly thought-out
hacks.

Believe
it or not, the above description is somewhat oversimplified, to aid
clarity. The air bypass valve (stock dump valve) has been left out since
all such OEM recirculating valves operate on similar grounds. Here is
a more complete
one with EGR, EVAP and other hose connections.

On
top of all these static relationships, the rpm ranges are rough and
they do vary according to the position of the throttle
and also the rate of it's movement. For example the ECU orders the IACV
open a bit after the EGCV has opened. If the throttle is fully open
however, this rev 'gap' is deliberately shortened compared to that of
mild acceleration.